Technical Field
Embodiments of the invention relate generally to heat transfer. Other embodiments relate to heat sinks for cooling transistor modules.
Discussion of Art
Certain semiconductor devices, such as a BJT, diode, IGBT, or MOSFET, can be used as switches to control a flow of current. Semiconductor devices can be bipolar, in which both holes and electrons carry charge across a junction in opposite directions; or unipolar, in which just one type of charge carrier (electron or hole) is used to carry charge across the junction. Some semiconductor devices are triggered by briefly supplying a small current across or adjacent the junction, such as transistors or thyristors. Other semiconductor devices are switched on by applying a constant bias voltage across the junction, such as FET, MOSFET, or IGBT. While triggered or switched on, semiconductor device switches carry current generally proportional to a junction area.
Manufacturing processes limit the achievable junction or channel area of the semiconductor chips on which electronic devices are built. Thus, in order to conduct an arbitrary current under a given voltage, an arbitrary number of chips may be packaged together as a module.
When a semiconductor device conducts electrical current, heat is produced by internal resistance of the device. When the heating rate of a module (having plural such semiconductor devices) exceeds the available heat dissipation, there is a risk of temperature rise affecting changes in electrical behavior, such that thermal damage could occur to portions of the module. Accordingly, it is important to provide adequate cooling for a semiconductor module, and to equalize temperatures as much as possible among chips comprising a semiconductor module.
Often, semiconductor modules are cooled by air convection, either forced or natural. Convective cooling rates are generally proportional to temperature difference, air velocity, and surface area. It may be desirable to maintain temperature difference as low as possible, in other words, to keep the transistor module as cool as possible. Air velocity typically is limited by available fan power (which may be zero for natural convective cooling). Thus, to achieve a low temperature difference it is necessary to enhance the cooling surface area of a transistor module. For this purpose, a module may be mounted to a heat sink. A typical heat sink comprises a base plate with the module mounted to a first working surface of the base plate and with fins protruding opposite the first working surface to form a second working surface of the base plate. Often, a heat sink is described in terms of “thermal resistance,” which refers to a relationship of temperature difference to cooling rate.
Conventionally, thermal resistance is described using units of K/W. For example, a heat sink may be rated to provide 1 K/W thermal resistance from a first working surface to a coolant adjacent a second working surface. If heat is uniformly dissipated to the first working surface at a rate of 2 W, then the first working surface will be at 2 K above the temperature of coolant at the second working surface. However, thermal resistance values for heat sinks are misleading because thermal resistance quoted for a heat sink as a whole presumes uniform areal distribution of heat dissipation at the first working surface of the heat sink.
In real applications, heat dissipation is not areally uniform across a transistor module. For example, each semiconductor device within a module presents a discrete region of heating. Thus, if the transistors of a transistor module produce 5 W total heating and if the transistor module is mounted on a first working surface of a heat sink with 1 K/W overall thermal resistance, it is possible that temperatures at the first working surface of the heat sink, adjacent each of the transistors in the module, may be significantly more than 5 K above the coolant temperature adjacent a second working surface of the heat sink. Meanwhile, temperatures at the first working surface of the heat sink, distal from any of the transistors, may be significantly less than 5 K above the coolant temperature adjacent the second working surface.
Lateral heat transfer, within the base plate of a heat sink, tends to equalize temperatures across the first working surface of the heat sink, thereby bringing the heat sink closer to a uniform areal distribution of heat dissipation. Lateral heat transfer is limited by the thickness of the base plate, with a thinner base plate offering less lateral heat transfer, and less uniformity of heat dissipation. However, a thicker base plate presents greater thermal resistance from the first working surface to the second working surface, such that in designing an air-cooled heat sink for use with a transistor module, there is a tradeoff between achieving uniform heat dissipation and achieving minimum thermal resistance. Thus, air cooling constrains the achievable power through a module.